Lignocellulose-based anion-adsorbing medium (LAM) and process for making and using same for the selective removal of phosphate and arsenic anionic contaminants from aqueous solutions

ABSTRACT

A lignocellulose-based anion-adsorbing medium (LAM) and process for making and using same for selectively removing phosphates and arsenic contaminants from aqueous solutions is disclosed. Making the LAM comprises (a) dissociating cations such as Fe and Al, from their counterions by adding a chemical compound containing said cations to water and acidifying; (b) pelletizing a lignocellulose; (c) adsorbing the cations to the lignocellulose by bringing the lignocellulose into contact with the solution of step (a) and incubating; and, (d) exposing the lignocellulose of step (c) to an alkaline fixing agent to replace hydrogens (H) of the hydroxyl groups of the lignocellulose with the adsorbed cations to produce the LAM with a positive charge. The LAM may be used to selectively and cost-effectively remove phosphate and arsenic contaminants from aqueous solutions by retaining them at the Fe or Al on the LAM.

This application is a divisional of prior nonprovisional applicationSer. No. 10/708,001, filed Jan. 31, 2004, now U.S. Pat. No. 7,311,842.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is entitled to the benefit of Korean Patent Applicationserial number KR-10-2003-0064186, filed 2003 Sep. 16 by applicant JuYoung Kim and entitled: Manufacturing method of lignocellulose mediacoupled with Fe or Al.

BACKGROUND OF INVENTION

Anionic contaminants in wastewater and other aqueous solutions posesignificant environmental and health problems. For example, excessivelevels of phosphate in seawater, freshwater, wastewater and sewage causeundesirable biological effects such as red tides and eutrophication.Arsenic in groundwater and mine wastewater threatens the health andlives of human beings, animals and plants when it is consumed.Therefore, there have been many efforts to remove these anioniccontaminants from water.

With regard to phosphate removal, it is impossible to cost effectivelyremove this contaminant in cases of non-point source pollution, that is,in cases where the polluted water is drained from wide areas into riversand the sea. Non-point source pollution streams are typically highvolume streams with low concentrations of pollutants.

For point source pollution, where pollutants originate from specificpoints (e.g. industrial or domestic wastewater), current technologiesinclude chemical precipitation, biological treatment, MBR (membraneBio-coupled Reactor) method, ion exchange and absorption method.

The chemical precipitation method is widely used by small to mediumsized sewage treatment plants to remove phosphate. However, at lowconcentrations, the efficiency of phosphate removal is low. Even atconcentrations typical of domestic wastewater, removal efficiencies aretypically less than 60 percent. In addition, this method generates alarge amount of sludge, the disposal of which means extra cost.

Biological treatment is used by medium to large sized sewage treatmentplants to remove phosphate. This method employs a biologicalpretreatment prior to the addition of coagulation/precipitationchemicals. It suffers from similar disadvantages as the chemicalprecipitation method discussed above.

The MBR method makes use of ultrafiltration membranes in a reactordesign that allows for a continuous process as opposed to the batchwiseprecipitations in the methods described above. While this reactortechnology is much more efficient in removing phosphate, it shares withthe chemical precipitation and biological treatment technologies uponwhich it is based the disadvantage of producing concentrated sludgewhich must be removed from the reactor at intervals. It is also veryexpensive to install and maintain.

Ion exchange and reverse osmosis methods have been suggested as methodsto remove arsenic in groundwater and mine wastewater. Semiconductor,electronics, and dyeing plants use ion exchange resins to remove avariety of charged contaminants. Because of the extremely high cost andlimited capacity, wastewater pretreatment is required. This technologyis appropriate only for specialized industrial purposes, and not forphosphate or arsenic removal in point/non-point source pollution.Reverse osmosis is similarly unsuited for such applications because itis very expensive and difficult to maintain.

For the foregoing reasons, there is a need for a means of removinganionic contaminants from aqueous solutions that is both highlyefficient and cost-effective over a broad range of applications.

SUMMARY OF INVENTION

The present invention is directed to a lignocellulose-basedanion-adsorbing medium (LAM) and its processes for synthesis and usethat satisfy this need.

In one aspect of the present invention, a process for making thelignocellulose-based anion-adsorbing medium (LAM) with a positive chargeis disclosed, comprising the steps of (a) dissociating cations selectedfrom the group consisting of Fe and Al (or other divalent or trivalentcations with similar reactivity) from their counterions by adding achemical compound containing said cations to water and acidifying; (b)pelletizing a lignocellulose; (c) adsorbing the cations to thelignocellulose having hydroxyl groups (—OH) by bringing thelignocellulose into contact with the solution of step (a) andincubating; and, (d) replacing hydrogens (H) of the hydroxyl groups ofthe lignocellulose with the cations to produce the LAM with a positivecharge by incubating the lignocellulose of step (c) with an alkalinefixing agent.

In one version of the process for making the LAM, the anionic cationsare Fe or Al.

Alternatively, instead of the lignocellulose being pelletized prior tothe absorbing step (c), the LAM may be manufactured first usingunpelletized lignocellulose, and the LAM itself subsequently fashionedinto pellets.

In another aspect of the present invention, a process for treating acontaminated aqueous solution to remove one or more anionic contaminantstherefrom is disclosed and comprises contacting the contaminated aqueoussolution with a lignocellulose-based anion-adsorbing medium (LAM), madeaccording to the process herein disclosed, and recovering a treatedaqueous solution with reduced anionic contaminant content. The anioniccontaminants may consist of phosphate, arsenic or other anioniccontaminants. This process may further include regenerating the LAMafter its contact with the contaminated aqueous solution by treating theLAM with an alkaline solution to remove the one or more anioniccontaminants from the LAM and subsequently neutralizing the LAM with anacid to prepare it for reuse.

In one version of the method for using the LAM, the anion contaminantsare phosphates or arsenic.

The reader is advised that this summary is not meant to be exhaustive.Further features, aspects, and advantages of the present invention willbecome better understood with reference to the following description,accompanying drawings and appended claims.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the present invention, reference may bemade to the accompanying drawings, in which:

FIG. 1 a depicts a cellulose reacted with Fe to form a LAM in which theFe replaces the hydroxyl H of the cellulose;

FIG. 1 b depicts a cellulose reacted with Al to form a LAM in which theAl replaces the hydroxyl H of the cellulose;

FIG. 2 a depicts a lignin reacted with Fe to form a LAM in which the Fereplaces the hydroxyl H of the lignin coating;

FIG. 2 b depicts a lignin reacted with Al to form a LAM in which the Alreplaces the hydroxyl H of the lignin coating;

FIG. 3 a presents the results of the adsorption experiment described inExample 6 using non-pelletized media; and,

FIG. 3 b presents the results of the adsorption experiment described inExample 6 using pelletized media.

DETAILED DESCRIPTION

Overview

The composition of the present invention is a lignocellulose-basedanion-adsorbing medium (LAM) that can be used to remove anioniccontaminants such as phosphate, arsenic or the like from aqueoussolutions in a highly efficient and cost-effective manner. The capacityof the LAM, i.e. the amount of contaminant removed per mass of media,exceeds that of existing technologies. The efficiency of removal, i.e.the level to which contaminants' concentrations can be lowered, issuperior to competing technologies. Sludge is not produced. The LAM canbe regenerated and reused multiple times. Further, the LAM ismanufactured by a simple and inexpensive process from lignocellulose, asafe and harmless natural material that is readily available at a verylow cost.

The present invention discloses a process for making the LAM such thatit will effectively and selectively remove anionic contaminants (e.g.,phosphate and arsenate) from aqueous solutions (e.g., drinking water,sea water, freshwater, sewage, groundwater, and mine wastewater).Lignocellulose is commonly defined as a combination of lignin, celluloseand hemicellullose that strengthens plant cells. The term lignocelluloseas it is used herein refers broadly to plant tissue, both woody tissuessuch as aspen and pine wood, and nonwoody tissues such as cotton andkenaf; to the main chemical constituents of plant tissue, such ascellulose, hemicellulose, starch, sugars, and lignin; and to products,preparations, and byproducts that contain the above referenced chemicalconstituents or their reaction products, such as paper, dextran, rayon,and pulping waste liquors. The lignocellulose is chemically modified soas to bestow it with a high density of positive charges. This isachieved by replacement of the hydroxyl hydrogens (H) of thelignocellulose with positively charged chemical moieties such as iron(Fe), aluminum (Al), calcium (Ca) or other divalent or trivalent cationswith similar reactivities which may include, e.g., Mg, Mn, Mo, Co, Ni,or Zn.

One implementation of the invention is realized as follows. A chemicalcompound containing Fe or Al is added to water and the Fe or Al isdissociated from its counterion by adding acid. Next, lignocellulose isadded to the above solution, where it adsorbs Fe or Al. Finally, analkaline fixing solution or fixing gas is used to catalyze the chemicalreplacement of lignocellulose's hydroxyl H with Fe or Al. The result isa positively-charged lignocellulose-based anion-adsorbing medium (LAM).

The adsorption capacity of the LAM manufactured by this invention isgreater than that of currently used media. In addition, theeffectiveness and efficiency of anion contaminant removal by use of LAMis greater than that exhibited by other current technologies that arenot media-based. Further, the LAM is highly selective for arsenic andphosphate. For example, LAM can be used to effectively remove arsenic ingroundwater and mine wastewater, and phosphate in seawater, freshwater,and sewage. Further, by the regeneration process described herein, LAMcan be regenerated to its original adsorption capacity and used for manyadsorption-regeneration cycles, leading to great cost effectiveness.

Detailed Description Process for Making LAM.

This invention provides a process for making a lignocellulose-basedanion-adsorbing medium (LAM) by which lignocellulose is chemicallymodified so as to bestow it with a high density of positive charges.This is achieved by replacement of the hydroxyl hydrogens (H) oflignocellulose with positively charged chemical moieties. For the sakeof simplicity, the following describes the steps for achieving this forthe specific cases in which LAM is manufactured using Fe or Al; however,it should be noted that similar results may be obtained by use of otherpositively charged chemical moieties such as, e.g., Mg, Ca, Mn, Mo, Co,Ni and Zn.

The steps by which a lignocellulose-based anion-adsorbing medium (LAM)with a positive charge is manufactured are, (a) dissociating cationsfrom their counterions by adding a chemical compound containing saidcations to water and acidifying; (b) pelletizing a lignocellulose; (c)adsorbing the Fe or Al cations to a lignocellulose having hydroxylgroups (—OH) by bringing the lignocellulose into contact with thesolution of step (a) and incubating; and, (d) incubating the treatedlignocellulose from the adsorbing step (c) with an alkaline fixingsolution or gas that catalyzes the replacement of hydrogens (H) of thehydroxyl groups of the lignocellulose with Fe or Al cations to producethe LAM with a positive charge.

(a) Dissociating cations from their counterions by adding a chemicalcompound containing said cations to water and acidifying.

A chemical compound containing Fe or Al (or other divalent or trivalentcations with similar reactivity which may include, e.g., Ca, Mg, Mn, Mo,Co, Ni, Zn or the like) is added to water. Some examples of chemicalcompounds containing Fe or Al are: FeI₂, FeCl₂, FeCl₃, FeBr₂, FeBr₃,FeF₂, FeF₃, FeSO₄, Fe₂(SO₄), Fe(NO₃)₃, FePO₄, AlI₃, AlCl₃, AlBr₃, AlF₃,AlSO₄, Al₂(SO₄)₃, Al(NO₃)₃, AlPO₄ and the like. This solution isacidified to the extent required to dissociate Fe or Al and to maintainthem in the dissociated form. The concentration of Fe or Al andcorresponding volume employed is chosen by reference to considerationswell-known to those skilled in the art of chemistry to ensure that asufficient but not overabundant amount of Fe or Al will be adsorbed ontothe lignocellulose added in the adsorbing step (c) to bring about anefficient coupling reaction in step (d). For example, FeCl₃ or AlCl₃ at0.01 3.0 M (molar concentration) dissociated by addition of acids suchas HCl, H₂SO₄, HNO₃, and so on at 0.1 1.0 N (normal concentration) areeffective conditions.

(b) Pelletizing a lignocellulose. Pelletized LAM may be made frompelletized lignocellulose media. In that case, the lignocellulose ispelletized prior to use at the adsorbing step (c).

Pellets may be manufactured from a variety of lignocelluloses, includingwood, cotton fiber, liquified cellulose, and other lignocellulosicmaterials. For example, pelletized lignocellulose media may be producedby rolling a sheet of paper tightly until it makes a stick of properdiameter and pasting the end of the paper with a non-toxic, insolubleglue. The rolled paper stick may then be cut into suitable lengths. Inthis way pelletized lignocellulose media of various diameters andlengths may be made.

In another version, pellets may be constructed from cotton strings. Thestring is cut into suitable lengths, and a non-toxic insoluble glue isused to seal the cut ends and prevent unraveling. Because cottoncontains fats that may interfere with the LAM-manufacturing process,pretreatment to remove these fats should be performed. Fats may beremoved by treatment with an organic solvent or by boiling in water orin a slightly acidic aqueous solution.

In another version, pellets may be derived by milling naturallignocelluloses, e.g. corn cobs, and screening to obtain an appropriatesize distribution of pellets. Natural pellets must be prewashed toremove easily leached components (hemicelluloses, soluble lignin, freesugars, etc.) prior to their use in the manufacture of LAM. Althoughthese leachable components are in general non-toxic, they wouldinterfere with the coupling of Fe or Al to non-leachable sites bycompetition and by physically blocking access to these sites.

The exact process whereby a LAM is manufactured from lignocellulosicpellets will vary depending upon the physical characteristics of thepellet. For example, to achieve optimal penetration of Fe or Al (step(c), below), incubation time must be varied inversely with pelletporosity. In addition, less porous pellets may demand higher vacuums foradequate penetration. Because of the demanding kinetics of the fixationstep (step (d), below), incubation time of fixation may not besubstantially varied; therefore, proper vacuum or pressure conditions(see step (d), below) are important.

The reader should note that, as an alternative to convertinglignocellulosic pellets to LAM, it is possible in some cases to firstconvert the lignocellulose (unpelletized) to LAM and subsequently togenerate the pellets out of the LAM product. For example, paper (inunpelletized form) may be first converted to LAM and then the LAMproduct subsequently pelletized by the process described above forpelletizing lignocellulose. Likewise, cotton fibers (unpelletized) mayfirst be converted to LAM and the LAM product subsequently pelletized.This approach may obviate somewhat the problems discussed above withregard to achieving optimal penetration into the pellet.

(c) Adsorbing the cations to a lignocellulose having hydroxyl groups(—OH) by bringing the lignocellulose into contact with the solution ofstep (a) and incubating.

Lignocellulose is added to the above solution and incubated for asufficient time to allow it to adsorb adequate quantities of Fe, Al, Caor other cations used. As an alternative to dipping the lignocelluloseinto a vessel containing treatment solution, the solution may be appliedby spraying it onto the lignocellulose. This can be very efficient whenspraying occurs at high pressure. A vacuum treatment may also beemployed at this step to facilitate penetration of the reagent into thelignocellulose matrix.

Vacuum treatment may be employed to remove pockets of gas from thelignocellulose undergoing treatment. These air pockets could otherwiseblock access of Fe or Al from many potential sites of attachment. Thehigh capacity of LAM is based upon the even distribution of cations(i.e., Fe, Al, Ca or others) at a molecular level by virtue of the evendistribution of the hydroxyl groups of lignocellulose. Therefore, anoptimized vacuum treatment is important. The strength and duration ofthe vacuum treatment depend upon the physical characteristics of thelignocellulose undergoing treatment. For example, if the lignocelluloseis in the form of a thin sheet of material, e.g. paper, then a brieftreatment under low vacuum is sufficient, whereas if the lignocelluloseis in a hardened pelletized form, more substantial treatment isrequired.

A wetting pretreatment with a water-miscible solvent whose surfacetension is less than that of water, e.g methanol, ethanol, etc., may beperformed on the lignocellulose prior to addition of the Fe or Alsolution and application of vacuum. The lower surface tension of theprewetting solvent results in the formation of smaller air bubbles whichmore readily outgas. During incubation, a two-step process results inthe nano-scale distribution of the Fe or Al. First, air bubbles areremoved by the vacuum treatment. Second, the wetting solution isdisplaced by diffusion and dilution into the far larger aqueoustreatment solution, bringing the Fe or Al into contact with thelignocellulosic hydroxyl groups. Alternatively, the surface tension ofthe treatment solution itself may be minimized by inclusion of asurfactant.

In some cases, vacuum treatment may not be required, but may be replacedwith other treatment regimes which likewise have the effect ofdisplacing trapped gases from the lignocellulose. Examples ofalternative treatments include, e.g., prewetting alone, heating,boiling, sonication, pressurization, etc. Following this incubationstep, the lignocellulose is typically dried prior to performance of step(d).

(d) Incubating the treated lignocellulose from the adsorption step (c)with an alkaline fixing solution or gas that catalyzes the replacementof hydrogens (H) of the hydroxyl groups of the lignocellulose withcations to produce the LAM with a positive charge.

A fixing solution or fixing gas is used to catalyze the chemicalreplacement of lignocellulose's hydroxyl H with cations (i.e., Fe, Al,Ca or others). An alkaline solution such as NaOH, KOH, Ca(OH)₂ or NH₄OHmay be used for this purpose. For example, fixation may be achieved byincubation for 0.1 to 60 minutes, typically 0.2 5.0 minutes, in asolution of NH₄OH at 1.0 8.0 M.

A vacuum treatment may be employed to facilitate penetration of thesolution into the lignocellulose matrix. Unreacted Fe or Al is removedfrom the LAM immediately after the prescribed incubation period byrinsing with water.

The considerations regarding use of a vacuum treatment are identical tothose discussed above for the adsorbing step (c); however the kineticsof the fixation reaction pose additional technical difficulties. Thestep (d) treatment differs from the step (c) treatment in that theduration of incubation of the former must be carefully controlled. Ifthe incubation period is too short, the coupling of the cations (Fe, Al,Ca or others) to the lignocellulosic hydroxyl groups will be incomplete,and a LAM of suboptimal capacity will be generated. If the incubationperiod is too long, excess cations (Fe, Al, Ca or others) willflocculate or participate within the lignocellulose matrix and blockaccess to the molecularly dispersed cations (Fe, Al, Ca or others) thatconstitute the active removal sites, likewise resulting in a LAM withsuboptimal capacity.

These demanding kinetics are the reason a treatment to dry thelignocellulose at the conclusion of the adsorption step (c) may berequired. The rapid penetration of the fixing solution into thelignocellulose matrix is facilitated by the wicking properties of driedlignocellulose. For the case of wet lignocellulose, penetration mustoccur by diffusion, which may be too slow to bring the treatment intocontact with all relevant sites within the narrow optimized timeframerequired. For these reasons, a drying treatment is typically performedat the conclusion of the adsorption step (c).

As an alternative to incubation with an alkaline solution, fixation maybe achieved by incubation with an alkaline gas. For example, fixationmay be achieved by incubation for 0.5 to 120 minutes, typically 2 to 10minutes, with NH₄OH gas at 1.0 8.0 M. A drying treatment is typicallyperformed at the conclusion of the adsorbing step (c); otherwise,penetration would be limited by the rate of dissolution of the activeagent into the wet matrix. Typically, a pressure treatment to facilitateoptimal penetration of the fixing gas into the lignocellulose matrix isemployed.

The fixing reaction is quenched by addition of an excess of water.Excess Fe or Al that was not fixed to the lignocellulose is washed outby rinsing with water. The LAM is then ready for use in the removal ofanionic contaminants.

By way of illustration, the reactions of Fe and Al with cellulose arediagrammed in FIGS. 1 a and 1 b respectively. The hydroxyl hydrogens ofcellulose are replaced with Fe or Al to form —OFe or —OAl moieties.Hemicelluloses may be modified in similar fashion. Because of theextremely dense arrangement of hydroxyl groups in cellulose andhemicellulose, the reaction generates a solid substrate with anextremely high charge density.

The reactions of Fe and Al with lignin are diagrammed in FIGS. 2 a and 2b, respectively. In this case, the groups subject to modification arephenolic hydroxyls.

EXAMPLES

The process for manufacturing LAM (Fe form) from nonpelletizedlignocellulose was optimized as follows. Aqueous solutions of FeCl₃ranging in concentration from 0.01 to 3.0 M were prepared. Thesesolutions were adjusted to HCl concentrations ranging from 0.1 to 1.0 N.170 mg paper (25×25×1 mm) was added to each solution and incubated for0.1 2.0 hours. Paper samples were removed from these solutions anddried. Dried paper samples were then fixed with 1.0 8.0 M NH₄OH for 0.110 minutes, after which the unreacted Fe was removed by rinsing withwater. Vacuum treatments were employed during cation loading andfixation incubations.

The manufacture of LAM (Fe form) from pelletized lignocellulose wasoptimized as described above. The pelletized lignocellulose mediaconsisted of a sheet of paper tightly rolled and glued into the form ofa stick with a diameter of 2 mm and then cut into 5 mm long segments ofabout 30 mg each. The rest of the experiment is the same as describedabove except that it used pelletized media instead of nonpelletizedlignocellulose media.

The manufacture of LAM (Al form) from nonpelletized and pelletizedlignocellulose was accomplished as described above, except that aqueoussolutions of AlCl₃ ranging in concentration from 0.01 to 3.0 M wereused.

Detailed Description Process for Using LAM.

The process for treating a contaminated aqueous solution to remove ananionic contaminant such as phosphate, arsenic or the like therefrom,comprises contacting the contaminated aqueous solution with alignocellulose-based anion-adsorbing medium (LAM), wherein the LAM hasbeen formed by the steps disclosed above, and recovering a treatedaqueous solution with reduced anionic contaminant content. The methodsmay further consist of regenerating the LAM after its contact with thecontaminated aqueous solution by treating the LAM with an alkalinesolution to remove the one or more anionic contaminants from the LAM andsubsequently neutralizing the LAM with an acid to prepare it for reuse.

Contacting the contaminated aqueous solution with a lignocellulose-basedanion-adsorbing medium (LAM), wherein the LAM has been formed by thesteps disclosed above, and recovering a treated aqueous solution withreduced anionic contaminant content.

The LAM, in either its pelletized or non-pelletized form, may be placedin contact with contaminated aqueous solutions on both small and verylarge scales. Once the contaminated aqueous solution comes into contactwith the LAM, the anionic contaminants (e.g. phosphate or arsenic) areretained at the Fe or Al on the LAM. In this way, the concentrations ofthese contaminants in the aqueous solution are lowered through contactwith the LAM. The aqueous solution may then be circulated back to itsoriginal location, be it a small contained area or a larger body ofwater, or alternatively transferred to an alternative location for a usebefitting its cleansed state.

The selectivity of LAM for arsenic and phosphate makes it extremelyuseful for the removal of these contaminants from waters which maycontain a variety of other anions. This selectivity is an especiallyvaluable characteristic for the removal of, for example, lowconcentrations of arsenic from groundwaters with very substantial levelsof anions such as, e.g., chloride, sulfate or carbonate. In such asituation, the capacity of a nonselective anion-adsorbing medium for thetarget contaminant would be rapidly depleted by the binding of thecompeting anions. However, the selectivity of LAM is such that thesepotential competitors are not bound. Thus, even huge excesses of otheranions such as, e.g., chloride, sulfate or carbonate do notsubstantially reduce the number of LAM binding sites that are availableto remove arsenic and phosphate, and therefore remediation of thesechallenging waters can be achieved.

Different forms of LAM may be used as appropriate based upon the natureof the contaminated solution, the scale of the operation, and theavailability of technological infrastructure. In the followingdiscussion, capacity refers to the mass of a contaminant that a givenmass of LAM will adsorb. Efficiency refers to the percentage of removalof a given contaminant by a given configuration of LAM. Note that a highcapacity does not necessarily translate into a high efficiency: a givendesign could theoretically remove large quantities of a contaminant byremoving only, for example, 50% of the contaminant from a high volumetreatment stream. For the case of the removal of arsenic from drinkingwater, depending of course on the initial concentration of the water tobe treated, high efficiencies may be required to reduce concentrationsto levels safe for human consumption. Whereas capacity is a fundamentalproperty of a given type of medium, efficiency is influenced both by theproperties of the medium and the design of the system employing it.

Specific examples of the use of LAM to remove arsenic and phosphate fromvarious aqueous media are described below.

Regarding arsenic removal, a preoxidation step may be of benefit forcertain treatment streams. Nearly all treatment technologies exhibit agreater affinity for arsenic in its oxidized state, As(V), than in itsreduced state, As(III). However, it is noteworthy that the highlyefficient removal of even arsenic as As(III) is achieved by use of LAM.The development of other operational parameters, such as pH, bed volume,flow rate, etc., will be easily addressed by those skilled in the art.

LAM may be used to remove arsenic from groundwater, a non-point sourceapplication. A single-family point-of-use system in a technologicallyadvanced setting would make use of LAM engineered to optimize capacityand removal efficiencies. For this situation, a membrane design would beoptimal. The high head loss across the membrane could be easily overcomeby pressurization. Contaminated water would be forced through smallpores in a LAM matrix. This configuration maximizes the ratio of LAMsurface area to treatment stream, thereby maximizing capacity andefficiency. Pre-filtration would be required to prevent fouling of theLAM system.

While such a membrane design could conceivable be used to remove arsenicin a small-scale water treatment plant as well, the scale of such asystem is limited to relatively low volume applications because of thehigh head loss across the system. Pelletized LAM would be used in highervolume applications.

Pelletized LAM is used in a packed bed configuration. The size of thepelletized particle is important. Head loss and fouling problems arereduced with an increase in particle size; unfortunately, mediumcapacity and efficiency are likewise reduced with an increase inparticle size. LAM beds may be operated in series or in parallel. Theformer configuration increases the efficiency of the system, but limitsflow rates. Parallel operation increases throughput but has no effect onefficiency, unless it makes possible an accompanying reduction in theflow rate of the treatment stream.

Small particles of LAM (e.g., 1.0-10.0 mm) in a static bed design wouldbe an appropriate design for water treatment plants of various sizes.Pre-filtration and pressurization would be required to prevent foulingand generate practical flow rates. Such a design could be used forsingle-family point-of-use applications as well.

Similar designs could also be used to treat point source arsenic wastestreams such as mining wastes. Here, the economic trade-off would bebetween LAM capacity and pre-treatment cost. LAM in a membrane or smallparticle form provides more capacity than large particles would, butdemands a more rigorous pre-treatment of the waste stream.

Pelletized LAM of larger particle sizes (ca. 10-30 mm) have lesscapacity, but can be utilized without pre-treatment and pressurization.Because of the relatively large interstitial spaces in the medium bed,particulates pass through with the treatment stream without fouling theLAM bed. Further, because of the low head loss across such a bed, agravity feed design is feasible. This configuration of LAM would beuseful in systems to remove arsenic from drinking water in developingcountries, where in many cases the technologies discussed above are notavailable. A baffled sedimentation tank removes large particulates, andwater for drinking is gravity fed from this tank through a bed oflarge-particle LAM. Such a design could be used for community orsingle-family systems.

An even simpler point-of-use solution for arsenic removal is gravityfiltration of water for drinking directly into a pitcher. A reservoir atwhose outlet is placed a cartridge of LAM is set atop a collectingvessel. The user then simply pours water to be treated into thereservoir and it gravity feeds into a pitcher.

In the case of phosphate removal, point sources may be treated usingdesigns similar to those described above for water treatment plants. Inmost cases, waste streams of extremely high initial phosphateconcentrations are first pre-treated to reduce these concentrations, andLAM would be employed in a final polishing step to further reduceconcentrations to acceptable discharge levels.

LAM could be used in the treatment of animal waste generated by feedlotoperations. Initial treatments may consist of anaerobic fermentation orditch oxidation, with LAM employed to treat the aqueous effluent fromthese processes. Because the discharge requirements for release ofphosphate into the environment are much less demanding than those forremoval of arsenic in drinking water, efficiency is of less importancein this application. Therefore, to avoid fouling and reduce technologycosts, pelletized LAM of relatively large particle size would likely bethe medium of choice.

Large-particle LAM is the medium of choice for remediation of urbannon-point source phosphate pollution and of natural surface waters aswell. In the former case, pelletized LAM would be packed into thedistribution boxes of storm sewers, or installed in other locations inthe flow path. Under normal conditions of low velocity flow, thistreatment would substantially lower phosphate levels prior to discharge.

Remediation of rivers, lakes, and bays present huge problems of scale,but such undertakings are nevertheless justified when phosphate-driveneutrophication results in severe problems. Phosphate removal may beachieved by recirculating contaminated water through large static bedsof LAM. In the case of rivers and bays, where mixing may be inadequateto allow for effective treatment by this means, the entire area may beexposed to LAM by use of a large mesh of LAM suspended from boats.

Regenerating the LAM after its contact with the contaminated aqueoussolution by treating the LAM with an alkaline solution to remove one ormore anionic contaminants from the LAM to which the anionic contaminantshave been adsorbed, and subsequently neutralizing the LAM with an acidto prepare it for reuse. Though this process may be used to selectivelyremove either phosphate or arsenic contaminants, it should be understoodby the reader that there will inevitably be some other anioniccontaminants (i.e., non-phosphate or non-arsenic) adsorbed as well.These other adsorbed anionic contaminants will also be removed from theLAM by this regeneration process.

The LAM will accumulate anionic contaminants with use. Eventually, allavailable Fe or Al sites will be occupied by anions, with the resultthat no additional anions can be removed from the contaminated solutionbeing treated. At this point, the capacity of the LAM has been exceeded,and break-through of contaminants occurs.

Depleted LAM may be regenerated by eluting the adsorbed anioniccontaminants (e.g., phosphate, arsenic and/or other anioniccontaminants) with an alkaline solution and then neutralizing theregenerated LAM with acid. Alkaline solutions such as NaOH, KOH, Ca(OH)₂and NH₄OH, and acid solutions such as HCl, H₂ SO₄ and HNO₃ can be usedfor this purpose.

In the case of phosphate, elution with NaOH yields a “waste” product inthe form of an alkaline phosphate fertilizer, a product with a positivevalue. Thus, when used to remove phosphate, the process is better than azero discharge process in that it actually produces a product with apositive economic value.

In the case of arsenic, a low volume of highly concentrated arsenic isobtained, which must most probably be disposed of as toxic waste. It ispossible that in some cases selective desorption treatments may be usedto obtain a highly purified arsenic solution that would have a positivevalue; however, this may depend upon the characteristics of the wastestream.

The efficacy of LAM in removing phosphate and arsenic from variousaqueous media are described below. The described results were obtainedby testing Fe-loaded LAM.

The ability of LAM to remove phosphorus in the form of phosphate fromwater was tested as follows. 50 mL aliquots of water were brought to aconcentration of 10.0 mg/L phosphorus with phosphoric acid and thenincubated with LAM for 24 hours with shaking at 150 rpm. For assessmentof nonpelletized LAM, a single 170-mg sheet was tested. For assessmentof pelletized LAM, six 30-mg pellets were tested.

Batch treatment with nonpelletized LAM resulted in a lowering of thephosphorus concentration in water from 10.0 mg/L to 0.06 mg/L (>99%efficiency). In addition to demonstrating that removal of phosphate tovery low levels can be achieved, this finding also demonstrates that thecapacity of the LAM employed exceeds 2.95 mg phosphorus per gm LAM. Thepelletized LAM treatment lowered the phosphorus concentration in waterfrom 10.0 mg/L to 0.50 mg/L (95% efficiency), thereby demonstrating acapacity of at least 2.64 mg phosphorus per gm for the pelletized LAM.

The ability of LAM to remove phosphorus as phosphate from artificialseawater (Instant Ocean, Synthetic sea salt, Nitrate-Free,Phosphate-Free, Aquarium Systems Inc.) was tested by the same protocol.Batch treatment resulted in a lowering of the phosphorus concentrationin seawater from 10.0 mg/L to 2.8 mg/L (72% efficiency) by use ofnonpelletized LAM and to 2.5 mg/L (75% efficiency) using pelletized LAM.Given the preponderance of competing anions in seawater, this findingdemonstrates the selectivity of LAM for the targeted contaminant.

The ability of LAM to remove arsenic from a complex matrix in which itis commonly found was assessed. The matrix chosen was leachate from woodtreated with CCA (chromated copper arsenic, a common wood preservative).CCA leachates contain arsenic in both its As(III) and As(V) oxidationstates. The test matrix was generated by extracting CCA-treated woodwith water and adjusting the extract by dilution to give an arsenicconcentration of ca. 10 mg/L. These arsenic solutions were treated withLAM as described above for the phosphate tests. Treatment withnonpelletized LAM lowered the arsenic concentration in the leachate from10.6 mg/L to 0.05 mg/L (>99% efficiency) and treatment with pelletizedLAM lowered it to 0.3 mg/L (>97% efficiency). This finding demonstratesthat removal of arsenic from a complex matrix to very low levels can beachieved, and that the capacity of the LAM employed exceeds 3.0 mgarsenic per gm LAM. These findings are particularly significant withregard to the high capacity and efficiency of removal of arsenic in itsAs(III) oxidation state: in nearly all other treatment technologies,removal of As(III) is much more problematic than that of As(V).

Commercial adsorbants were tested using the same protocols as describedabove for LAM testing. Results are compared to those obtained with LAMin FIGS. 3 a and b. The reader should note that the true capacities ofLAM exceed those shown in FIGS. 3 a and b by virtue of the fact that(with the exception of removal of phosphate from seawater) contaminantconcentration was the limiting factor in the LAM experiments. For thecommercial adsorbants, contaminant concentration was in no caselimiting; therefore, the data shown do represent these adsorbents' truecapacities.

To determine whether phosphate loaded LAM would leach bound phosphate,ca. 175 mg LAM loaded with ca. 3 mg phosphorus as phosphate per gram wasincubated with shaking in 50 mL water for 24 hours. No phosphorus wasdetected in the leachate (detection limit=ca. 0.001 mg/L). A similarexperiment with arsenic loaded LAM resulted in no detectable arsenic inthe leachate. The highly irreversible nature of these associations hasobvious commercial implications, among them a utility of LAM incontinuous flow applications.

LAM may be regenerated to its original capacity by a simple treatmentwith alkali. This procedure was demonstrated by treating phosphateloaded LAM for ten minutes in NaOH solutions ranging in concentrationfrom 0.01% to 10.0%. These LAM samples were then rinsed two to threetimes in water and neutralized using 1 N HNO₃. The ability ofregenerated LAM to remove phosphate from water or seawater wasessentially the same as that reported above for LAM that had not beenpreviously used. Similar experiments with arsenic loaded LAM likewisedemonstrated a complete restoration of capacity. The number ofregeneration cycles that may be used depends on the characteristics ofthe lignocellulose used to manufacture the LAM. For example, whereas thestructural characteristics of pelletized paper LAM were adverselyaffected by several cycles of use and regeneration, pelletized cottonLAM exhibited no such ill effects.

All the items discussed above are detailed explanations of the inventionthrough examples for illustration only; therefore, they do not restrictthe invention to themselves. Many other variations and modifications ofthe invention will be apparent to those skilled in the art withoutdeparting from the spirit and scope of the invention. Theabove-described embodiments are, therefore, intended to be merelyexemplary, and all such variations and modifications are intended to beincluded within the scope of the invention as defined in the appendedclaims. For example, lignocellulose may be modified in a similar fashionby use of other elemental salts containing Mg, Ca, Mn, Mo, Co, Ni, orZn, in addition to the Fe and Al described above. In addition, solidsubstrates other than lignocellulose may be employed, if they containhydroxyl groups susceptible to the replacement reaction described abovewhereby LAM is manufactured.

Advantages

The LAM manufactured by this invention has a greater absorption capacitythan that of any commercial absorption media, and is capable ofproducing effluents with substantially lower concentrations than thoseachievable by commercial media. Further, it is highly selective forarsenic and phosphate, making it practical to use in treatment of waterswith high levels of other anions whose removal is not desired and whichwould only serve to decrease the capacity of the LAM. In addition, LAMefficiently removes As(III), which is problematic for nearly all othertreatment technologies. Potential applications include but are notlimited to removal of arsenic in groundwater and mine wastewater andremoval of phosphate in freshwater, wastewater and sewage. Not only doesLAM provide a simple yet effective treatment process for point sourcepollution, its capabilities indicate that it could be used with greatsuccess to remove phosphate or arsenic in cases of non-point sourcepollution, for which there are currently no practical treatment methods.

Moreover, LAM may be manufactured from natural materials such asperennial plants (trees) and annual plants (kenaf, rice straw andcotton), completely safe and harmless materials that will not generatesecondary pollution. Because of the low costs of all manufacturinginputs, and of the simplicity of the manufacturing process, LAM is verycost effective to produce. Further, because it can be simply regeneratedto its original absorption capacity and used multiple times, its use ismade more economical still.

Closing

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference.

All the features disclosed in this specification (including anyaccompanying claims, abstract, and drawings) may be replaced byalternative features serving the same, equivalent or similar purpose,unless expressly stated otherwise. Thus, unless expressly statedotherwise, each feature disclosed is one example only of a genericseries of equivalent or similar features.

1. A process for treating a contaminated aqueous solution to remove oneor more phosphate anionic contaminants therefrom, comprising: a.contacting the contaminated aqueous solution with a lignocellulose-basedanion-adsorbing medium (LAM), wherein the LAM has been formed by thesteps of: i. dissociating cations selected from the group consisting ofFe, Al and Ca, from their counterions by adding a chemical compoundcontaining said cations to water and acidifying; ii. pelletizing alignocellulose having hydroxyl groups (—OH); iii. adsorbing the cationsto a lignocellulose having hydroxyl groups (—OH) by bringing thelignocellulose into contact with the solution of step (i) andincubating; and iv. replacing hydrogens (H) of the hydroxyl groups ofthe lignocellulose with the cations to produce the LAM with a positivecharge by exposing the lignocellulose of step (iii) to an alkalinefixing agent; and, b. recovering a treated aqueous solution with reducedcontent of the phosphate anionic contaminants.
 2. The method of claim 1,further comprising regenerating the LAM after its contact with thecontaminated aqueous solution by treating the LAM with an alkalinesolution to remove the phosphate anionic contaminants from the LAM towhich the phosphate anionic contaminants have been adsorbed, andsubsequently neutralizing the LAM with an acid to prepare it for reuse.3. The method of claim 1, wherein the lignocellulose is selected fromthe group consisting of wood, paper, and cotton.